Flux<String> + "application/json" renders as text

Spring MVC now treats Flux<String> + "application/json" as (serialized)
text to be written directly to the response as is. This is consistent
with the rendering of String + "application/json".

Issue: SPR-15456

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandler.java

@@ -131,16 +131,16 @@ class ReactiveTypeHandler {
 				new SseEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
 				return emitter;
 			}
+			if (CharSequence.class.isAssignableFrom(elementClass)) {
+				ResponseBodyEmitter emitter = getEmitter(mediaType.orElse(MediaType.TEXT_PLAIN));
+				new TextEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
+				return emitter;
+			}
 			if (mediaTypes.stream().anyMatch(MediaType.APPLICATION_STREAM_JSON::includes)) {
 				ResponseBodyEmitter emitter = getEmitter(MediaType.APPLICATION_STREAM_JSON);
 				new JsonEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
 				return emitter;
 			}
-			if (CharSequence.class.isAssignableFrom(elementClass) && !isJsonStringArray(elementClass, mediaType)) {
-				ResponseBodyEmitter emitter = getEmitter(mediaType.orElse(MediaType.TEXT_PLAIN));
-				new TextEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
-				return emitter;
-			}
 		}
 
 		// Not streaming...
@@ -162,12 +162,6 @@ class ReactiveTypeHandler {
 				this.contentNegotiationManager.resolveMediaTypes(request) : mediaTypes;
 	}
 
-	@SuppressWarnings("OptionalUsedAsFieldOrParameterType")
-	private boolean isJsonStringArray(Class<?> elementType, Optional<MediaType> mediaType) {
-		return CharSequence.class.isAssignableFrom(elementType) && mediaType.filter(type ->
-				MediaType.APPLICATION_JSON.includes(type) || JSON_TYPE.includes(type)).isPresent();
-	}
-
 	private ResponseBodyEmitter getEmitter(MediaType mediaType) {
 		return new ResponseBodyEmitter() {
 

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandlerTests.java

@@ -16,10 +16,13 @@
 package org.springframework.web.servlet.mvc.method.annotation;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.List;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Collectors;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -52,8 +55,8 @@ import org.springframework.web.servlet.HandlerMapping;
 import static junit.framework.TestCase.assertNull;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
+import static org.springframework.core.ResolvableType.forClass;
 import static org.springframework.web.method.ResolvableMethod.on;
 
 /**
@@ -108,27 +111,27 @@ public class ReactiveTypeHandlerTests {
 
 		// Mono
 		MonoProcessor<String> mono = MonoProcessor.create();
-		testDeferredResultSubscriber(mono, Mono.class, () -> mono.onNext("foo"), "foo");
+		testDeferredResultSubscriber(mono, Mono.class, forClass(String.class), () -> mono.onNext("foo"), "foo");
 
 		// Mono empty
 		MonoProcessor<String> monoEmpty = MonoProcessor.create();
-		testDeferredResultSubscriber(monoEmpty, Mono.class, monoEmpty::onComplete, null);
+		testDeferredResultSubscriber(monoEmpty, Mono.class, forClass(String.class), monoEmpty::onComplete, null);
 
 		// RxJava 1 Single
 		AtomicReference<SingleEmitter<String>> ref = new AtomicReference<>();
 		Single<String> single = Single.fromEmitter(ref::set);
-		testDeferredResultSubscriber(single, Single.class, () -> ref.get().onSuccess("foo"), "foo");
+		testDeferredResultSubscriber(single, Single.class, forClass(String.class), () -> ref.get().onSuccess("foo"), "foo");
 
 		// RxJava 2 Single
 		AtomicReference<io.reactivex.SingleEmitter<String>> ref2 = new AtomicReference<>();
 		io.reactivex.Single<String> single2 = io.reactivex.Single.create(ref2::set);
-		testDeferredResultSubscriber(single2, io.reactivex.Single.class, () -> ref2.get().onSuccess("foo"), "foo");
+		testDeferredResultSubscriber(single2, io.reactivex.Single.class, forClass(String.class), () -> ref2.get().onSuccess("foo"), "foo");
 	}
 
 	@Test
 	public void deferredResultSubscriberWithNoValues() throws Exception {
 		MonoProcessor<String> monoEmpty = MonoProcessor.create();
-		testDeferredResultSubscriber(monoEmpty, Mono.class, monoEmpty::onComplete, null);
+		testDeferredResultSubscriber(monoEmpty, Mono.class, forClass(String.class), monoEmpty::onComplete, null);
 	}
 
 	@Test
@@ -137,13 +140,15 @@ public class ReactiveTypeHandlerTests {
 		// JSON must be preferred for Flux<String> -> List<String> or else we stream
 		this.servletRequest.addHeader("Accept", "application/json");
 
-		EmitterProcessor<String> emitter = EmitterProcessor.create();
-		testDeferredResultSubscriber(emitter, Flux.class, () -> {
-			emitter.onNext("foo");
-			emitter.onNext("bar");
-			emitter.onNext("baz");
+		Bar bar1 = new Bar("foo");
+		Bar bar2 = new Bar("bar");
+
+		EmitterProcessor<Bar> emitter = EmitterProcessor.create();
+		testDeferredResultSubscriber(emitter, Flux.class, forClass(Bar.class), () -> {
+			emitter.onNext(bar1);
+			emitter.onNext(bar2);
 			emitter.onComplete();
-		}, Arrays.asList("foo", "bar", "baz"));
+		}, Arrays.asList(bar1, bar2));
 	}
 
 	@Test
@@ -153,48 +158,17 @@ public class ReactiveTypeHandlerTests {
 
 		// Mono
 		MonoProcessor<String> mono = MonoProcessor.create();
-		testDeferredResultSubscriber(mono, Mono.class, () -> mono.onError(ex), ex);
+		testDeferredResultSubscriber(mono, Mono.class, forClass(String.class), () -> mono.onError(ex), ex);
 
 		// RxJava 1 Single
 		AtomicReference<SingleEmitter<String>> ref = new AtomicReference<>();
 		Single<String> single = Single.fromEmitter(ref::set);
-		testDeferredResultSubscriber(single, Single.class, () -> ref.get().onError(ex), ex);
+		testDeferredResultSubscriber(single, Single.class, forClass(String.class), () -> ref.get().onError(ex), ex);
 
 		// RxJava 2 Single
 		AtomicReference<io.reactivex.SingleEmitter<String>> ref2 = new AtomicReference<>();
 		io.reactivex.Single<String> single2 = io.reactivex.Single.create(ref2::set);
-		testDeferredResultSubscriber(single2, io.reactivex.Single.class, () -> ref2.get().onError(ex), ex);
-	}
-
-	@Test
-	public void jsonArrayOfStrings() throws Exception {
-
-		// Empty -> null
-		testJsonNotPreferred("text/plain");
-		testJsonNotPreferred("text/plain, application/json");
-		testJsonNotPreferred("text/markdown");
-		testJsonNotPreferred("foo/bar");
-
-		// Empty -> List[0] when JSON is preferred
-		testJsonPreferred("application/json");
-		testJsonPreferred("application/foo+json");
-		testJsonPreferred("application/json, text/plain");
-		testJsonPreferred("*/*, application/json, text/plain");
-	}
-
-	private void testJsonNotPreferred(String acceptHeaderValue) throws Exception {
-		resetRequest();
-		this.servletRequest.addHeader("Accept", acceptHeaderValue);
-		EmitterProcessor<String> processor = EmitterProcessor.create();
-		ResponseBodyEmitter emitter = handleValue(processor, Flux.class);
-		assertNotNull(emitter);
-	}
-
-	private void testJsonPreferred(String acceptHeaderValue) throws Exception {
-		resetRequest();
-		this.servletRequest.addHeader("Accept", acceptHeaderValue);
-		EmitterProcessor<String> processor = EmitterProcessor.create();
-		testDeferredResultSubscriber(processor, Flux.class, processor::onComplete, Collections.emptyList());
+		testDeferredResultSubscriber(single2, io.reactivex.Single.class, forClass(String.class), () -> ref2.get().onError(ex), ex);
 	}
 
 	@Test
@@ -211,14 +185,10 @@ public class ReactiveTypeHandlerTests {
 
 		// No media type preferences
 		testSseResponse(false);
-
-		// Requested media types are sorted
-		testJsonPreferred("text/plain;q=0.8, application/json;q=1.0");
-		testJsonNotPreferred("text/plain, application/json");
 	}
 
 	private void testSseResponse(boolean expectSseEimtter) throws Exception {
-		ResponseBodyEmitter emitter = handleValue(Flux.empty(), Flux.class);
+		ResponseBodyEmitter emitter = handleValue(Flux.empty(), Flux.class, forClass(String.class));
 		assertEquals(expectSseEimtter, emitter instanceof SseEmitter);
 		resetRequest();
 	}
@@ -228,7 +198,7 @@ public class ReactiveTypeHandlerTests {
 
 		this.servletRequest.addHeader("Accept", "text/event-stream");
 		EmitterProcessor<String> processor = EmitterProcessor.create();
-		SseEmitter sseEmitter = (SseEmitter) handleValue(processor, Flux.class);
+		SseEmitter sseEmitter = (SseEmitter) handleValue(processor, Flux.class, forClass(String.class));
 
 		EmitterHandler emitterHandler = new EmitterHandler();
 		sseEmitter.initialize(emitterHandler);
@@ -238,11 +208,11 @@ public class ReactiveTypeHandlerTests {
 		processor.onNext("baz");
 		processor.onComplete();
 
-		assertEquals("data:foo\n\ndata:bar\n\ndata:baz\n\n", emitterHandler.getOutput());
+		assertEquals("data:foo\n\ndata:bar\n\ndata:baz\n\n", emitterHandler.getValuesAsText());
 	}
 
 	@Test
-	public void writeSentEventsWithBuilder() throws Exception {
+	public void writeServerSentEventsWithBuilder() throws Exception {
 
 		ResolvableType type = ResolvableType.forClassWithGenerics(ServerSentEvent.class, String.class);
 
@@ -258,7 +228,7 @@ public class ReactiveTypeHandlerTests {
 		processor.onComplete();
 
 		assertEquals("id:1\ndata:foo\n\nid:2\ndata:bar\n\nid:3\ndata:baz\n\n",
-				emitterHandler.getOutput());
+				emitterHandler.getValuesAsText());
 	}
 
 	@Test
@@ -266,8 +236,8 @@ public class ReactiveTypeHandlerTests {
 
 		this.servletRequest.addHeader("Accept", "application/stream+json");
 
-		EmitterProcessor<String> processor = EmitterProcessor.create();
-		ResponseBodyEmitter emitter = handleValue(processor, Flux.class);
+		EmitterProcessor<Bar> processor = EmitterProcessor.create();
+		ResponseBodyEmitter emitter = handleValue(processor, Flux.class, forClass(Bar.class));
 
 		EmitterHandler emitterHandler = new EmitterHandler();
 		emitter.initialize(emitterHandler);
@@ -275,19 +245,22 @@ public class ReactiveTypeHandlerTests {
 		ServletServerHttpResponse message = new ServletServerHttpResponse(this.servletResponse);
 		emitter.extendResponse(message);
 
-		processor.onNext("[\"foo\",\"bar\"]");
-		processor.onNext("[\"bar\",\"baz\"]");
+		Bar bar1 = new Bar("foo");
+		Bar bar2 = new Bar("bar");
+
+		processor.onNext(bar1);
+		processor.onNext(bar2);
 		processor.onComplete();
 
 		assertEquals("application/stream+json", message.getHeaders().getContentType().toString());
-		assertEquals("[\"foo\",\"bar\"]\n[\"bar\",\"baz\"]\n", emitterHandler.getOutput());
+		assertEquals(Arrays.asList(bar1, "\n", bar2, "\n"), emitterHandler.getValues());
 	}
 
 	@Test
 	public void writeText() throws Exception {
 
 		EmitterProcessor<String> processor = EmitterProcessor.create();
-		ResponseBodyEmitter emitter = handleValue(processor, Flux.class);
+		ResponseBodyEmitter emitter = handleValue(processor, Flux.class, forClass(String.class));
 
 		EmitterHandler emitterHandler = new EmitterHandler();
 		emitter.initialize(emitterHandler);
@@ -297,31 +270,35 @@ public class ReactiveTypeHandlerTests {
 		processor.onNext("the lazy dog");
 		processor.onComplete();
 
-		assertEquals("The quick brown fox jumps over the lazy dog", emitterHandler.getOutput());
+		assertEquals("The quick brown fox jumps over the lazy dog", emitterHandler.getValuesAsText());
 	}
 
 	@Test
-	public void writeTextContentType() throws Exception {
+	public void writeFluxOfString() throws Exception {
+
+		// Default to "text/plain"
+		testEmitterContentType("text/plain");
 
-		// Any requested, concrete, "text" media type
+		// Same if no concrete media type
+		this.servletRequest.addHeader("Accept", "text/*");
+		testEmitterContentType("text/plain");
+
+		// Otherwise pick concrete media type
 		this.servletRequest.addHeader("Accept", "*/*, text/*, text/markdown");
 		testEmitterContentType("text/markdown");
 
-		// Or any requested concrete media type
+		// Any concrete media type
 		this.servletRequest.addHeader("Accept", "*/*, text/*, foo/bar");
 		testEmitterContentType("foo/bar");
 
-		// Or default to...
-		testEmitterContentType("text/plain");
-
-		// Or default to if not concrete..
-		this.servletRequest.addHeader("Accept", "text/*");
-		testEmitterContentType("text/plain");
+		// Including json
+		this.servletRequest.addHeader("Accept", "*/*, text/*, application/json");
+		testEmitterContentType("application/json");
 	}
 
 	private void testEmitterContentType(String expected) throws Exception {
 		ServletServerHttpResponse message = new ServletServerHttpResponse(this.servletResponse);
-		ResponseBodyEmitter emitter = handleValue(Flux.empty(), Flux.class);
+		ResponseBodyEmitter emitter = handleValue(Flux.empty(), Flux.class, forClass(String.class));
 		emitter.extendResponse(message);
 		assertEquals(expected, message.getHeaders().getContentType().toString());
 		resetRequest();
@@ -329,9 +306,9 @@ public class ReactiveTypeHandlerTests {
 
 
 	private void testDeferredResultSubscriber(Object returnValue, Class<?> asyncType,
-			Runnable produceTask, Object expected) throws Exception {
+			ResolvableType elementType, Runnable produceTask, Object expected) throws Exception {
 
-		ResponseBodyEmitter emitter = handleValue(returnValue, asyncType);
+		ResponseBodyEmitter emitter = handleValue(returnValue, asyncType, elementType);
 		assertNull(emitter);
 
 		assertTrue(this.servletRequest.isAsyncStarted());
@@ -345,10 +322,6 @@ public class ReactiveTypeHandlerTests {
 		resetRequest();
 	}
 
-	private ResponseBodyEmitter handleValue(Object returnValue, Class<?> asyncType) throws Exception {
-		return handleValue(returnValue, asyncType, ResolvableType.forClass(String.class));
-	}
-
 	private ResponseBodyEmitter handleValue(Object returnValue, Class<?> asyncType,
 			ResolvableType genericType) throws Exception {
 
@@ -369,7 +342,9 @@ public class ReactiveTypeHandlerTests {
 
 		io.reactivex.Single<String> handleSingleRxJava2() { return null; }
 
-		Flux<String> handleFlux() { return null; }
+		Flux<Bar> handleFlux() { return null; }
+
+		Flux<String> handleFluxString() { return null; }
 
 		Flux<ServerSentEvent<String>> handleFluxSseEventBuilder() { return null; }
 	}
@@ -377,16 +352,20 @@ public class ReactiveTypeHandlerTests {
 
 	private static class EmitterHandler implements ResponseBodyEmitter.Handler {
 
-		private final StringBuilder stringBuilder = new StringBuilder();
+		private final List<Object> values = new ArrayList<>();
 
 
-		public String getOutput() {
-			return this.stringBuilder.toString();
+		public List<?> getValues() {
+			return this.values;
+		}
+
+		public String getValuesAsText() {
+			return this.values.stream().map(Object::toString).collect(Collectors.joining());
 		}
 
 		@Override
 		public void send(Object data, MediaType mediaType) throws IOException {
-			this.stringBuilder.append(data);
+			this.values.add(data);
 		}
 
 		@Override
@@ -406,4 +385,16 @@ public class ReactiveTypeHandlerTests {
 		}
 	}
 
+	private static class Bar {
+
+		private final String value;
+
+		public Bar(String value) {
+			this.value = value;
+		}
+
+		public String getValue() {
+			return this.value;
+		}
+	}
 }